Why You Should Feel Free to Ignore Polls for a Few Weeks

Donald Trump officially became the Republican party’s nominee Thursday night, and on Monday, the Democratic convention begins in Philadelphia. In the coming weeks, you can expect lots of polls — and headlines — suggesting new insight into the state of the presidential race.

With some caveats, our advice is: Don’t pay too much attention to them.

You can see what we mean in the chart above. It shows how much the polling average at each point of a presidential election cycle has differed from the final result. Each gray line represents a presidential election since 1980; the bright green line represents the average difference. In general, as the election nears, the polling average comes closer and closer to the election’s final result — but not for the next few weeks.

History suggests that in the short periods after the conventions, the polling average can often move away from the final result, not toward it. That’s because polls taken in the middle of the convention are often unreliable: Gains made by the party’s nominee can often be short-lived.

One approach is to ignore the polls during this tricky period. Our presidential forecast, which currently gives Hillary Clinton a 74 percent chance to win, does something different: It imposes a small penalty on polls taken when a candidate might be receiving a convention bounce. This penalty, based on conventions since 1980, fades out over the next few weeks.

At the same time, and perhaps more important, the model increases its variance estimate — how uncertain it is about its assessment of the race — during the convention period. Because of this volatility, it makes sense to judge polling over the next few weeks with more skepticism than usual. As a general rule, when variance goes up, it helps the underdog, so it’s possible that the model’s estimate of Mrs. Clinton’s overall chance of winning may decline slightly in the weeks ahead.

Of course, it’s also possible that Mr. Trump will not get a convention bounce at all, reflecting perhaps the most disorganized, unpredictable and bizarre convention in decades.

It’s unclear whether conventions still hold the power they once did. The bounces candidates received in 2008 and 2012 were more modest than some in years past, but television ratings for the conventions remain high. Ratings for the 2012 Democratic convention were the highest since 1992, and ratings for the Republicans in 2008 were the highest since 1976.

Bounce or no bounce, the history lesson remains: Polling averages tend to be volatile in the weeks after national conventions. As the election nears and the short-term effects of the conventions fade, the polling averages will rapidly become more precise.

In other words, if you’d like to take a break from political polls for a few weeks, you won’t be any worse for it. We’ll be here when you get back.

Until then, take a look at this series of charts showing how conventions have affected polling averages in every election since 1980.